The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Jan 1 PPL start still a possibility: PM

A January 1 start date for the federal government's changes to paid parental leave remains a possibility, but the prime minister says it is up for negotiation.

Malcolm Turnbull on Friday gave the strongest indication yet he is willing to move on the timing to ensure legislation clears the Senate.

Crossbench senators Derryn Hinch and Nick Xenophon are edging closer to an agreement with the coalition to help stop expectant mothers from "double-dipping", on the basis the proposed New Year's Day start date is delayed.

"That is our policy, so of course it's a possibility, but we have to negotiate with the crossbench senators and that's our job," Mr Turnbull told 3AW's Neil Mitchell on Friday.

"The different crossbench senators have expressed different views as to when it should start and we'll obviously be discussing that with them."

Mr Turnbull said the plan was an equitable and fair use of government resources that provided a safety net for mothers and fathers alike.